What would it feel like to fall into a black hole? This fanciful question has pit real physicists against each other for decades — and new research from Ohio State has opened up some old wounds.
“There’s a lot of people writing a lot of stuff about black holes and black hole information,” string theorist Nicholas Warner, Ph.D., a professor of physics and astronomy at the University of Southern California, tells Inverse. (Warner was not involved with the new study, by the way.)
“Some of it’s brilliant and wonderful and some of it is crap.”
If you go by the theory of general relativity, nothing escapes a black hole. Not even light travels fast enough to free itself from a black hole’s gravitational pull. It’s in the freaking name: A black hole is a spherical maw, floating in the void of space. It is a borg-like agglomeration of matter so dense that it assimilates everything that passes through its event horizon.
Unfortunately, none of this makes any sense from a quantum mechanical perspective. According to the rules, information simply can’t be destroyed, and yet that’s what black holes seem to do. This paradox has birthed a long-simmering intellectual war and, with the publication of the new study in the Journal of High Energy Physics, Ohio State physicists have recently turned up the heat.
“There was no simple fix in physics for this.”
The math at the core of quantum mechanics states that information cannot be obliterated, only converted into new forms: matter consumed into energy, energy forging new combinations of matter, and so on. But every time a complex, information-rich object is crushed into the uniform, impossibly compacted space of a black hole, all of its defining characteristics are lost — either flattened out or stretched into an infinitesimal spaghetti, depending on which theoretical model you choose. But no matter how you wish to think about it, the defining information encoded in that matter would appear to be totally obliterated.
For about a decade and change, Samir Mathur, Ph.D., a professor of physics at Ohio State, has been attempting to apply the higher-order, more multidimensional physical equations of string theory to this, the black hole information problem.
“In 2009, Samir Mathur proved a result about the black hole information paradox that showed it was much worse than we thought,” Warner recalls. “There was no simple fix in physics for this. It was going to require a large-scale rethinking of the physics of the event horizon.”
Mathur is an author on the new paper, an attempt to clarify his team’s string theory-based “fuzzball” resolution to the paradox. The only new problem is that it does so by throwing the controversial rival “firewall” theory under the bus.
In the wake of Mathur’s earlier research, four physicists at the University of California, Santa Barbara posited in 2012 that the seeming loss of information around an event horizon (as it does when, say, a genetically unique tree is burned into uniform gray ash) must come with the release of an incredible amount of energy. They proposed that, just underneath the surface of a black hole’s event horizon, there is a spherical “firewall” of energy ready to annihilate anything that dares to enter.
While it provided a working hypothesis for what might happen to all that supposedly lost information inside a black hole, more than a few scientists were unconvinced.
“I can tell you right now: the firewall idea is and always has been preposterous,” Chris Adami, Ph.D., a professor of microbiology, molecular genetics, physics, and astronomy at Michigan State University, tells Inverse.
Adami has been involved in this debate for a while. With a grant from the Army Research Office in 2003, he made an ambitious attempt to resolve the black hole information problem by testing it against some of the emerging principles of quantum communication theory.
Donald Marolf, Ph.D., one of the physicists that first proposed the firewall, waves off the criticisms. He does not believe that Adami’s research is even applicable to the debate because he thinks it misses the real issue at the heart of the black hole paradox: how to account for the “spontaneous emissions” proposed by none other than Stephen Hawking.
“Adami’s work discussed ‘stimulated emission’ by black holes,” Marolf tells Inverse. “But — despite common misconceptions — this has essentially nothing to do with the black hole info problem, which instead concerns the growth of [quantum] entanglement due to ‘spontaneous emission.’”
“I can tell you right now: the firewall idea is and always has been preposterous.”
Stephen Hawking, a professor of mathematics at the University of Cambridge, lecturing at an event honoring NASA's 50th Anniversary in 2008.
The paradox at the heart of the black hole information problem stems from Hawking radiation, an alleged spontaneous emission from black holes that Hawking justified theoretically in 1974.
The realization that a black hole would eventually die out after all these emissions, and the seemingly information-free quality of the Hawking radiation emissions, first indicated how fiercely black holes would bring relativity and quantum mechanics into conflict. Inside Hawking’s version of a black hole, it really looked like information was being destroyed.
Adami’s solution to the paradox addresses stimulated emissions — which contain more information than spontaneous emissions — but if you ask Marolf, that’s not enough to account for the information required to resolve the paradox. Word is still out, however, on Mathur’s theory.
In the classic interpretation of a black hole put forward by general relativity (top diagram), all of the black hole matter is constrained to a hyper-dense singularity at the center. In Mathur's string theory version, complex multidimensional geometries, the 'fuzzball,' emerge throughout the full volume of the black hole and approach tightly underneath the event horizon (bottom diagram).
There wasn’t always beef between these scientists.
In fact, for a while, Mathur’s fuzzball argument was wholly compatible with the firewall theory. He had shown through string theory that the interior of a black hole was not one, incredibly small, hyper-dense singularity surrounded by a powerful sucking void between it and the event horizon, but rather a volume filled with wiggling, multidimensional strings and branes — a “fuzzball” of information-rich materials vibrating out in dimensions beyond the three spatial dimensions, and time, that we experience as humans.
To string theorists like Mathur and Warner, the firewall theory addressed only the parts of the fuzzball that are very hot — in other words, it was not all-encompassing or mathematically rigorous, but it also wasn’t wrong.
“I would not say that the firewall argument was completely preposterous,” Mathur tells Inverse, “but the press did not report it accurately, so people were confused.”
Mathur’s new paper, however, takes a less accommodating approach to the firewall argument. At least, it’s no longer compatible with the idea that a person falling into a black hole would get burnt up in some kind of firewall, if it exists. Their explanation — due to some unique features of string theory — um, well, it’s deeply weird.
“This is the crucial part of the new physics,” Mathur says, “In the fuzzball theory, the inside of the black hole is not empty space; it is actually filled with a tangle of strings, branes, et cetera. When something falls into the hole, the hole gets bigger.”
“The question is: when exactly does it expand to its new size? We showed that new strings begin to form when the infalling object is still some distance from the surface of the original fuzzball. So, the infalling object never gets close enough to the original fuzzball surface to feel its heat — the newly formed strings change his infall behavior.”
Think of it as a hand approaching a mirror. Just as your hand can never touch a part of the mirror that isn’t a reflection of it, Mathur says, “similarly, the fuzzball surface moves up to meet the infaller, rather than wait for the infaller to reach near the surface and get burnt.”
Rather than feeling a burning sensation, or a shattering or crushing sensation, Mathur predicts that an ‘infaller’ would feel as though they were falling through a form of gently curved spacetime called “anti-de Sitter space.”
“This is the idea of ‘complementarity’,” Mathur says, “which the firewall people were trying to shoot down.” Understanding this is key to answering the inevitable question: What happens when you get sucked into a black hole?
The answer relies on the assumption that, in quantum mechanics, everything can be described by a set of frequencies. “If two systems have the same frequencies, then we cannot tell them apart. For example, if a piano and a keyboard emitted the same frequencies, you could not hear them and tell that they were really very different internally.”
Likewise, a person can be described by a set of frequencies. If that person falls into a black hole, Mathur explains, “he hits the fuzzball and sets the fuzzball into vibrations. But suppose the fuzzball vibrates with the same frequencies. Then the infaller has been absorbed, but in a sense he continues to live on in terms of the vibrations of the fuzzball. Thus he will not feel that he has been absorbed.”
(For a rough, maybe irresponsible, approximation, feel free to just imagine a person going through a black hole and then entering some psychedelic interstellar phantasia from an old Jack Kirby Marvel comic. Let’s say Doctor Strange.)
Multiple images of the 'real monster black hole' at the heart of the colliding galaxies Arp 299. At top, NASA's Nuclear Spectroscopic Telescope Array has recorded X-ray data projected out by the system. At middle, the NuSTAR high-energy X-ray data has been overlaid on a visible-light image from NASA's Hubble Space Telescope. The bottom panel shows Hubble's visible-light image alone.
Though Mathur’s explanation is satisfying (albeit complicated), it’s unclear whether it will put an end to the black hole paradox problem anytime soon. Neither Adami nor Marolf felt comfortable commenting on the new research from Mathur and his team, each for his own reasons. Warner would not either, but admits he feels very confident that Mathur is on the right track by applying string theory to the internal dynamics of black holes.
String theory has been notoriously airy and abstracted. It has been accused of being non-falsifiable, its adherents like religious zealots in their commitment to a series of mathematical equations that have, thus far, strongly suggested the existence of many dimensions outside the four we all deal with regularly — without a single physical experiment to confirm them.
But if we want to solve the black hole paradox once and for all, we may very well have to take their giant leap of faith.
“If I had to bet one the place where you might get some indication of string theory in the next decade or two,” Warner says, “it might just be in black hole physics.”